Recent capital expansion and upgrades in sheet metal mills have produced reducing mills which run faster and can take larger reductions in the sheet metal thickness during each pass. These passes are done in rapid succession, building up a high level of thermal energy in the sheet metal which is wound into coils without allowing much heat to dissipate.
According to conventional practice, an interleaver is typically co-wound into the windings of a sheet metal roll between the metal layers. The interleaver remains in contact with the sheet metal up to a week or more until the coil is fed into the cold annealing and pickling line wherein the interleaver is wound out of the coil.
The increased production rates achieved in many sheet metal mills have resulted in wound sheet metal temperatures as high as 200.degree. C. at the point the interleaver is introduced into the coil. At these temperatures, the natural kraft and laminated papers which have been used as interleavers in the past often do not perform satisfactorily, exhibiting thermal degradation and adhesion to the sheet metal which causes clouding or surface roughness on the metal. This adds cost to the process because the metal has to be further processed to remove the surface damage.
Because of these effects, many sheet metal mills have had to either slow down the process or add more oil to cool the sheet metal and to aid in release of the paper from the sheet metal. Each of these options adds cost to the process.
Japanese Patent No. 18199 describes a heat resistant laminated paper for use as an interleaver for sheet metal. The laminated paper contains 0.5 to 5.0 weight percent based on the weight of the pulp of a synthetic resin formed from polyacrylamide, urea and melamine. The heat resistance of the paper may be further improved by adding dicyandiamide to the paper. A significant disadvantage associated with the use of such resins is that they contain or result in release of formaldehyde, which is a respiratory irritant and a possible carcinogen. In addition, the use of dicyandiamide in the United States is restricted due to heath concerns and the material is expensive.
Many types of silicone coatings have been described for application to paper to improve various properties of the paper. U.S. Pat. No. 4,954,554 describes an aqueous polysiloxane emulsion which comprises a polyvinyl alcohol component as an emulsifying agent. The polysiloxane emulsion composition contains an organopolysiloxane bearing silicone-bonded curing radical selected from the group consisting of hydroxyl radicals and olefinic radicals, a polyvinyl alcohol emulsifying agent and water. The radicals of the curable organopolysiloxane include hexenyl radicals. Curing of the organopolysiloxane is achieved using a cross linking agent such as organoxhydrogenpolysiloxane.
U.S. Pat. No. 2,774,674 describes treatment of kraft paper with organopolysiloxanic oils to provide heat resistance and release properties. The organopolysiloxanic oils are prepared as an aqueous dispersion containing a mineral filler, an emulsifier and an alkali salt of an organosilane-triol. The organopolysiloxanic oils are polysiloxanes containing hydrocarbon groups such as alkyl, aryl, or aralkyl groups linked to a silicon atom and have a viscosity at 25.degree. C. between 100 and 1000 centistoke.
U.S. Pat. Nos. 4,190,688 and 3,463,661 describe silicone release coatings which are cured with heat and a catalyst. The '661 patent describes the use of a release coating prepared from polyvinyl alcohol, silicone resin, acetic acid, a wetting agent and a tin octoate catalyst. The '688 patent describes the use of a release coating prepared from a vinyl-containing siloxane polymer having hydroxy end groups. The polysiloxane polymer of the '688 patent is emulsified in water with polyvinyl alcohol with or without an organic solvent and a hydride polysiloxane cross-liking agent is used along with a tin or platinum catalyst at elevated temperatures to cure the coating.
Although the coatings described in the above patents have provided various improvements in paper properties, the high temperatures to which interleavers are exposed in modern sheet metal manufacturing coupled with the increased amount of oil used to address blocking problems has created a need for a further improved economical paper-based interleaver which will perform satisfactorily under these conditions. Standard coated paper grades with silicone-type coatings are relatively expensive and therefore do not adequately address this need, and formaldehyde-containing additives generally will not be accepted for environmental reasons.